The invention relates to a method for the production of tablets by means of a tablet press which includes a filling arrangement with a rotating filling wheel and metering wheel, the speed of rotation of which is adjustable, as well as an elevation-adjustable metering cam track on which the lower pistons of the tablet press ride, and an electronic measuring arrangement for the measuring of the pressing forces. In addition thereto, the invention relates to a tablet press.
In the production of tablets, it is desired that the tablets possess the same weight which corresponds, to the utmost extent, to the required weight and, beyond this, that the production of the individual tablets be performed under the influence of pressing forces which have a predetermined value and which are, to the largest possible extent, the same.
The pressing force is significant, among others, for the reason that the time of the dissolution or of the disintegration of the tablet depends on the same. When the pressing force is, for instance, too high, the tablet, under certain conditions, becomes dissolved when it has already left the stomach and is located in the intestine. The effect of such a tablet is different from that of a tablet having the same weight, but which has been produced under the influence of a lesser pressing force and, as a consequence thereof, dissolves more rapidly.
In addition thereto, the structural strength of the tablet depends on the pressing force, too. A certain structural strength of the tablet is required for the subsequent processing of the tablets, for example, in dragee vessels, and during the packaging. When the pressing force is, for instance, too low, the tablet will disintegrate even when subjected only to small loads. When the pressing force is too high, then the tablet is likely to disintegrate in layers. The experts refer to this phenomenon as "cleaving" of the tablets.
The weight of the tablets depends on the bulk density of the granulated material in the female die, as well as on the specific weight thereof. In addition thereto, the weight of the tablet depends on the filling amount, or on the free volume of the female die which is being filled with a granulated material as the female die, which is arranged in a rotating female die disk, passes through the filling arrangement. Therefore, the filling amount is determined by the position of the lower piston which extends into the female die from below and, in addition thereto, by the manner of the filling, or the speed of the filling of the female die.
For a complete filling of a female die during a speedy rotation of the female die disk, the conventional table presses possess a filling wheel and a metering wheel, which are being driven in opposite directions. Underneath the filling wheel, there is arranged a rail-like filling cam track and, underneath the metering wheel, a rail-like metering cam track which is situated at a somewhat higher elevation. These two cam tracks can be constructed as a one-piece rail which is arranged for adjustment of elevation (DT-OS No. 22 51 - 832). However, they also can be constructed in two parts, so that the metering cam track is elevationally adjustable with respect to the filling cam track as well.
When, during the production of a tablet, the corresponding female die enters the filling arrangement, the associated lower piston is being held at a lower elevation in the region of the filling wheel by the filling cam track, than in the succeeding region of the metering wheel in which it is being lifted by riding along the metering cam track. Therefore, after the granulated material for the production of the tablet has been filled into the female die by the filling wheel in the first partial section of the filling arrangement, a part of the same is upwardly expelled from the female die as a result of the lifting of the lower piston, and is wiped off by the metering wheel on the female die disk. This is accomplished in order to assure a complete filling of the female die. However, it has been established, especially in rapidly advancing tablet presses, that the bulk densities vary in a certain range when the speed of rotation of the filling and metering wheel is changed, or even the speed of rotation of the female die table. Thus, different filling amounts and, therefore, filling weights, result at different rotational speeds of the filling wheel. Consequently, the filling weights depend, on the one hand, on the controllable speed of rotation of the filling wheel and the metering wheel--both have, as a rule, the same speed of rotation--and on the elevational position of the lower piston, which can be influenced by an elevational adjustment of the filling and metering cam track, or exclusively of the metering cam track.
According to the DT-OS No. 22 51 832, it is known to compare the actual value of the pressing force being measured with a required value and to undertake a re-adjustment of the tablet weight in dependence thereon by an elevational adjustment of the metering cam track. By such an elevational adjustment of the metering cam track in itself, the tablet weight, it is granted, can be influenced more substantially than by a change in the rotational speed of the filling wheel. Despite this, the different speeds of rotation of the filling wheel are nevertheless of a considerable influence to the extent that, at certain speeds of rotation of the filling wheel, there result considerable weight variations between the tablets which are being produced in sequence while, on the other hand, the same tablet weights are obtainable only within a narrow range of the rotational speeds of the filling wheel. Therefore, it is expedient to utilize that speed of rotation of the filling arrangement which results in the lowest variations in the filling weight. However, this optimum speed of rotation is, in turn, again dependent of the size of the female die, or on the position of the lower piston.
In the mentioned conventional method of controlling the tablet weights under the utilization of the pressing forces, there is formed an average value of the pressing force from a plurality of pressing forces which have been determined in succession one after the other, which average value is compared with required limits and results in a cessation of operation of the machine when the latter are transgressed. Such a type of the control has not been shown to be complete. So, for instance, when the three determined values 10, 11, and 12, one after the other, are combined to an average value 11, and when the required value limits lie at 10 and 12, then the average value, as well as also all individual values, lie within these limits. However, when, under different assumptions, there are measured the values 1, 11, and 21, then there is also obtained, in fact, an average value 11 which is located between the permissible limits, even though, however, two of the three measured values are situated by far outside the permissible limits. Therefore, this known method does not take into consideration larger pressing force variations which do not find their reflection in an average value. On the other hand, if an undesired deviation of the actual value from the required value of the tablet weight was determined during the use of the known method, there resulted an automatic re-adjustment, or a manual one, by the elevational adjustment of the lower pistons or on the metering cam track. Pressing force variations, and thus tablet weight variations, which, accordingly, resulted, among others, from the respective rotational speed of the filling wheel, could not have, or have not, been taken into consideration therein. Besides, in any event, variations of the average value of the pressing force can be compensated for with time delays. As a consequence thereof, the variation width cannot be substantially influenced thereby.